1. Field of the Invention
The invention relates to methods for depolymerizing polysaccharides in which polysaccharides are reacted with hydroxyl free radicals to produce polysaccharides having lower molecular weights. The hydroxyl free radicals are formed by interaction of an oxidant with a catalyst comprising a ligand complexed with a metal ion, such as iron methylglycine diacetate or iron-2,3,4,5,6 pentahydroxyhexanoate. The invention also relates to a method for producing glucose. The invention further relates to a method for producing ethanol.
2. Description of the Related Art
Due to rising oil prices, there has been ever increasing interest in the use of ethanol as fuel. Ethanol is typically produced by fermenting sugars using certain species of yeast. The sugars are often obtained by hydrolyzing starches to produce sugars such as glucose. This hydrolysis of starch into glucose can be accomplished by treatment with an acid and/or enzymes. Currently, the most common source for the starches used in ethanol production is corn. However, there has been concern that the diversion of corn from food uses to ethanol production may cause unwanted price increases in food products including corn.
As a result, there has been interest in using alternative crops, such as agricultural residues, wood, and various grasses, in ethanol production. In these crops, cellulose is the source of sugars for fermentation to ethanol. However, compared to corn starch ethanol production, several factors make cellulosic ethanol production more costly and less efficient. First, more effort is needed to pretreat and solubilize hemicellulose and cellulose because they are locked into a rigid cell wall structure with lignin. Harsher thermochemical pretreatments may generate chemical by-products that inhibit enzyme hydrolysis and decrease the productivity of fermentative microbes. Second, the crystallinity of cellulose makes it more difficult for aqueous solutions of enzymes to convert cellulose to glucose.
Several processes have been proposed that could address the aforementioned disadvantages of cellulosic ethanol production. For example, processes have been developed that seek to improve the cellulase digestibility of cellulosic materials. U.S. Pat. No. 4,314,854 describes a process for enhancing the reactivity of cellulose-containing materials to cellulase enzymes by treating an aqueous suspension of the cellulose-containing material with hydrogen peroxide in the presence of a Mn+2 ion forming manganese compound. U.S. Pat. Nos. 4,806,475 and 4,649,113 describe a process in which agricultural crop residues and other nonwoody lignocellulosic plant substrates are treated with H2O2 such that the substrates are partially delignified and the products of the treatment have low crystallinity and near quantitative cellulase digestibility.
It has also been reported that hydroxyl radicals react with cellulose by cleaving bonds between glucose units in the polymer chain (see Cole et al., “Mechanisms of Oxidative Degradation of Carbohydrates During Oxygen Delignification I. Reaction of Photochemically Generated Hydroxyl Radicals with Methyl-b-D-Glucoside,” Journal of Wood Chemistry and Technology, 20:3, 2000; and Cole et al., “Mechanisms of Oxidative Degradation of Carbohydrates During Oxygen Delignification II. Reaction of Photochemically Generated Hydroxyl Radicals with Methyl-b-D-Cellobioside,” Journal of Wood Chemistry and Technology, 21:1, 2001; and Guay et al., “Mechanisms of Oxidative Degradation of Carbohydrates During Oxygen Delignification III. Reaction of Photochemically Generated Hydroxyl Radicals with 1,5-Anhydrocellobiotol and Cellulose,” Journal of Pulp and Paper Science, 28:7, 2002). Other uses of hydroxyl radicals and methods for generating hydroxyl radicals can be found in U.S. Pat. No. 6,960,330 to H. W. Cox, Jr.
However, there is still a need for alternative methods for depolymerizing cellulose or starch such that the cellulose or starch can be more easily hydrolyzed into sugars for fermentation into ethanol.